FIG. 13 is a circuit diagram illustrating the configuration of a conventional polyphase current supplying circuit. A single-phase ac power supply 21 supplies an ac voltage Vin to a diode bridge 11. However, an inductance parasitic to the power supply system is shown as an inductor 22 connected in series with the ac power supply 21.
The output of the diode bridge 11 is supplied to a smoothing circuit 12. The smoothing circuit 12 is composed only of a smoothing capacitor C having a small capacitance, e.g., several tens of microfarads. The smoothing capacitor C can be reduced in size because of its small capacitance.
A rectified voltage Vdc obtained across the smoothing capacitor C is input to an inverter 13. In the inverter 13, switching of transistors serving as switching devices is carried out on the basis of a switching instruction CNT obtained from a control circuit 14. Accordingly, three-phase currents iu, iv, iw are supplied to a motor 24. The control circuit 14 obtains the switching instruction CNT on the basis of the currents iu, iv, iw, a rotation position angle θm and a rotation angle speed (mechanical angle) ωm of a rotor of the motor 24, the ac voltage Vin, and the rectified voltage Vdc input to the inverter 13. These respective quantities iu, iv, iw, θm, ωm, Vin, Vdc can be detected using a well-known technique.
FIG. 14 is a graph showing the relationship between the input voltage Vin and rectified voltage Vdc, employing a time axis common to them on the horizontal axis. A capacitance of the smoothing capacitor C was set at 20 μF. Since the smoothing capacitor C has a small capacitance in this manner, its rectified voltage Vdc in turn has a very large pulsating component which pulsates at twice the frequency of the ac voltage Vin. Herein, the case in which the rectified voltage Vdc fluctuates between a little less than 300 V and a little more than 400 V is illustrated.
However, setting the switching instruction CNT as appropriate also allows control of switching in the inverter 13 in accordance with this pulsation, reduction of harmonics of current supplied from the ac power supply 21 to the diode bridge 11 and improvement in power factor on the power supply side.
A technique of controlling an inverter with a smoothing capacitor significantly reduced in capacitance in this manner will herein be called single-phase capacitorless inverter control. The single-phase capacitorless inverter control allows size reduction of a smoothing capacitor as described above as well as eliminating the necessity for using a reactor for improving the power factor, which can achieve size reduction of a polystage current supplying circuit as a whole to achieve cost reduction.
Prior art documents disclosing such single-phase capacitorless inverter control are Patent document 1 and Non-patent document 1.
Patent document 1: Japanese Patent Application Laid-Open No. 2002-354826
Non-patent document 1: Isao Takahashi “Inverter Controlling Method using PM Motor having High-Input-Power-Factor Diode Rectifier Circuit”, 2002 The Institute of Electrical Engineers of Japan, National Conference, 4-149 (March 2002), p. 1591